In the fourth generation mobile communication systems where video and/or data are mainly communicated, there is need of a far higher capability than in the third generation mobile communication systems (IMT-2000), and higher capacity, faster speed and broadbanding have to be achieved. In these systems, it is supposed that they are used under various communication environments such as inside and outside houses. Outside houses, plural cells (multi-cell) covering a wide area are provided for enabling fast packet transmission for a fast-moving mobile station. Inside houses, since radio waves are attenuated more drastically, access points are provided within buildings without support of radio communication at outdoor base stations. From other viewpoints such as improved utilization efficiency of communication resources, packet transmission type communication has been employed even in radio intervals rather than conventional circuit switching type communication. In communications between a mobile station and an upper apparatus other than a base station, particularly in downlink data transmission, not only a unicast scheme but also a multicast scheme and a broadcast scheme are employed. For example, see non-patent document 1 for an outlook of future communication systems.
On the other hand, frequency selective fading under multipath environments has significant influence in wideband mobile communication systems. Hence, the OFDM (Orthogonal Frequency Division Multiplexing) scheme is promising as the next generation communication scheme. In the OFDM scheme, a single symbol is generated by attaching a guard interval part to an effective symbol part including information to be transmitted, and plural symbols are transmitted during a predefined transmission time interval (TTI). The guard interval part consists of a portion of information within the effective symbol part. The guard interval part may be also called a cyclic prefix (CP) or overhead.
At the receiver side, paths are received with various propagation delays. According to the OFDM scheme, if the amount of propagation delay falls within the period of the guard interval part, inter-symbol interference can be effectively reduced. Thus, a relatively large guard interval period allows delay waves to be advantageously synthesized. This is advantageous particularly in communications with an extremely large cell radius and in simultaneous transmission of the same information from different cells to a mobile station in accordance with the multicast scheme. However, the guard interval part includes only a portion of the effective symbol part, and thus a larger period of the guard interval part is not preferred from the viewpoint of information transmission efficiency. In some cases, satisfactory communication quality may be maintained under environments with relatively short propagation delay such as urban areas and indoor areas or environments available for the unicast scheme by setting a relatively short guard interval part. Therefore, it is impossible to determine a single type of guard interval part optimized under various communication environments. For this reason, it may be conceived that many sets of radio parameters for specifying symbols including guard interval parts with various sizes are provided and radio communications are carried out in the adaptively determined optimal symbol format. However, signal processing corresponding to such various symbol formats leads to an extremely heavy workload, which is unfavorable for mobile stations with a relatively simple configuration. For a mobile station having no option of operating frequency (clock frequency), strictly limited signal processing is available, and thus the above problem may have a particularly adverse effect on such a mobile station.
Non-patent document 1: Ohtsu, “Systems beyond IMT-2000”, ITU Journal, Vol. 33, No. 3, pp. 26-30, March 2000